Salesforce Web To Lead form storing strange data

Question

I have some web to lead forms with a hard coded firstname and lastname, the records in Salesforce itself seem to randomly either be correctly showing the hard-coded fields or showing a random string such as 5c375c95c90d2 - all the web to lead form accounts should show the same "Firstname Lastname" for example, as this is simply a newsletter sign up prospect list.

What would cause the data to come through as the above from Salesforce's end, if anything?

<form action="https://webto.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8" method="POST" class="form subscribe" id="salesforce-signup-form" target="hiddeniFrame">

    <input type="hidden" id="###" name="###" value="UK">

    <input type="hidden" id="###" name="###" value="Sign Up">

    <input type="hidden" id="###" title="Channel" name="###" value="Direct">

    <input type="hidden" id="lead_source" name="lead_source" value="Web">

    <input id="first_name" maxlength="40" name="first_name" size="20" type="hidden" value="Firstname">

    <input id="last_name" maxlength="80" name="last_name" size="20" type="hidden" value="Lastname">

    <input type="hidden" id="Campaign_ID" name="Campaign_ID" value="###"><br>

    <input name="email" type="email" id="email" placeholder="Email address..." maxlength="80">

    <button class="action subscribe primary inactive" title="Submit" type="submit">
        <span>Submit</span>
    </button>

    <input type="checkbox" name="###" class="checkbox" id="###" value="1">

</form>